The art of diagnostic imaging exploits targetting agents that in binding or localizing sites selectively within the body, help to resolve the image of diagnostic interest. Monoclonal antibodies for example have been developed to have high affinity and specificity for particular cancer cells and therefore are useful for imaging tumours. Despite high affinity and specificity, antibodies do not provide ideal imaging agents since they are costly to produce on a commercial scale as well as their poor labelling characteristics. In particular, metal labels tend to bind at numerous low-affinity binding sites on antibodies and are released in vivo resulting in undesirable accumulation of the label at non-target sites. An alternative targetting agent to antibodies are small receptor binding peptides. Peptides offer the advantage of efficient labelling facilitated by conjugation to various chelating molecules. Other advantages of peptides over antibodies is their ease of synthesis, rapid tissue penetration and rapid clearance from the body.
A naturally occurring tetrapeptide, tuftsin TKPR (Seq. ID no: 1), was discovered to stimulate phagocytosis by binding to receptors expressed on the outer surface of neutrophils and macrophages. Phagocytosis constitutes a major line of defense for a host against bacterial infections, therefore as a stimulator of phagocytosis tuftsin would be expected to be a good peptide for imaging sites of infectious inflammation. However, studies show that tuftsin labelled with a radionuclide metal undesirably accumulates in non-target tissue. In particular, labelled tuftsin accumulated in the gastrointestinal tract which limits its usefulness as an imaging agent.
In light of the difficulties associated with antibodies, it would be desirable to provide a peptidic targetting agent capable of localizing at sites of inflammation while not having substantial accumulation in non-target tissue.